[Bug middle-end/78904] zero-extracts are not effective

["segher at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78904

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-12-22
     Ever confirmed|0                           |1

--- Comment #1 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Before combine we have

===
(insn 9 7 10 2 (set (subreg:SI (reg:QI 97) 0)
        (zero_extract:SI (reg/v:SI 95 [ b ])
            (const_int 8 [0x8])
            (const_int 8 [0x8]))) "78904.c":12 105 {*extzvsi}
     (expr_list:REG_DEAD (reg/v:SI 95 [ b ])
        (nil)))
(insn 10 9 11 2 (set (subreg:SI (reg:QI 98 [ a$val ]) 0)
        (zero_extract:SI (reg/v:SI 94 [ a ])
            (const_int 8 [0x8])
            (const_int 8 [0x8]))) "78904.c":12 105 {*extzvsi}
     (nil))
(insn 11 10 26 2 (parallel [
            (set (reg:QI 88 [ _2 ])
                (plus:QI (reg:QI 97)
                    (reg:QI 98 [ a$val ])))
            (clobber (reg:CC 17 flags))
        ]) "78904.c":12 214 {*addqi_1}
     (expr_list:REG_DEAD (reg:QI 98 [ a$val ])
        (expr_list:REG_DEAD (reg:QI 97)
            (expr_list:REG_UNUSED (reg:CC 17 flags)
                (nil)))))
(insn 13 12 14 2 (set (zero_extract:SI (reg:SI 92 [ D.1801 ])
            (const_int 8 [0x8])
            (const_int 8 [0x8]))
        (subreg:SI (reg:QI 88 [ _2 ]) 0)) "78904.c":14 109 {insvsi_1}
     (expr_list:REG_DEAD (reg:QI 88 [ _2 ])
        (nil)))
===

and combine then does

===
Trying 10, 9 -> 11:
Failed to match this instruction:
(parallel [
        (set (reg:QI 88 [ _2 ]) 
            (plus:QI (subreg:QI (zero_extract:SI (reg/v:SI 94 [ a ])
                        (const_int 8 [0x8])
                        (const_int 8 [0x8])) 0)
                (subreg:QI (zero_extract:SI (reg/v:SI 95 [ b ])
                        (const_int 8 [0x8])
                        (const_int 8 [0x8])) 0)))
        (clobber (reg:CC 17 flags))
    ])
===

and then it later tries 10, 9 -> 11 and 9, 11 -> 13 etc., but you need
11, 10, 9 -> 13 to match your insn pattern.

Maybe you want a pattern for the result of 10, 9 -> 11?  It is a valid
insn for i386 I think?